Very very preliminary read of the SSCI report. In October 2002 and January 2003 -- the January 2003 NIE we knew about -- the intelligence community circulated to the Bush administration several analyses warning that postwar Iraq would most likely be a nightmare.

A post-Saddam authority would face a deeply divided society with a significant chance that domestic groups would engage in violent conflict with each other unless an occupying force prevented them from doing so. ...

Use of violence by competing factions in Iraq against each other or the United States -- Sunni against Shia; Kurd against Kurd; Kurd against Arab; any against the United States -- would also encourage terrorist groups to take advantage of a volatile security environment to launch attacks within Iraq. ...

Some elements in the Iranian government could decide to try to counter aggressively the U.S. presence in Iraq or challenge U.S. goals following the fall of Saddam by attempting to use their contacts in the Kurdish and Shia communities to sow dissent against the U.S. presence and complicate the formation of a new, pro-U.S. Iraqi government. ...

Needless to say, there are a ton of political reasons why these warnings weren't heeded, not least of which that they cast serious doubt on the wisdom of invading Iraq. But there's another reason that might be even more determinative in a strict sense. From the SSCI's introduction:

Current and former intelligence officials told the Committee that intelligence reporting did not play a significant role in developing assessments about postwar Iraq because it was not an issue that was well-suited to intelligence collection. Accordingly, most prewar assessments cite relative few intelligence sources. Analysts based their judgments primarily on regional and country expertise, historical evidence and analytical tradecraft.

And you just can't have that, as Frank demonstrated quite excellently. "Regional and country expertise, historical evidence and analytical tradecraft" were considered the tools of a hidebound, unreliable and disloyal intelligence apparatus.